One form of macerator as shown, for example, in GB-A-1569672, includes first and second parallel contra rotating shafts, each having a plurality of alternate cutters and spacers of the same axial thickness, the cutters of the first shaft being interleaved with those of the second shaft. Each cutter has a plurality of teeth arranged around its periphery and circumferentially spaced locations.
Mounted adjacent to the cutters, on the side walls of the housing, are side rails which have radially inner surfaces which are arcuate and closely adjacent to the teeth of the cutters as they rotate. While this has proved to be reasonably satisfactory, the flow rate, particularly of a liquid-borne, in particular water-borne, material to be macerated is reduced rather severely because of the general blockage provided by the cutters and the side rails.
It has been proposed according to US-A-4702422 to provide slotted side rails in which the side rails themselves are formed with a plurality of parallel ribs which extend in the same circumferential direction as the cutters and have formed therebetween a plurality of slots. These slots face the spacers and the ribs face the cutters in operation. The slots provide a passage for fine material which does not in fact need to be macerated and for the water and hence the flow rate through the macerating apparatus is significantly increased as compared with that of GB-A-1569672.
However a real problem exists in that it is very often desirable that some of the materials which may be in sheet form which can pass through the slots should be cut by the teeth of the macerator. The slots can thus provide an undesirable bypass flow for material which should be macerated and hence the macerated product can have a rather larger cross section than it should and sheet materials are not necessarily allowed to be macerated because of the fairly large space which is available for them to flow through the apparatus.
It is a primary object of the present invention to overcome this problem while still reducing the blockage which has occurred in earlier macerators.